The invention relates to passive electromagnetic wave limiters as well as to duplexers which can be constructed by means of these limiters.
Passive limiters are generally formed by diodes whose performances reflect those which may then be expected from the duplexing devices which comprise them. Precise details are given in "Radar Handbook" by M. SKOLNIK, 1970 p. 8.24.
In fact, the passive limiters of the invention must be able to withstand ultra-high frequency peak powers of the order of a few hundred kilowatts.
In electromagnetic detection or radar equipment to which the present invention more particularly applies, the protection of the receiver of the radar against transmissions from its own transmitter or from neighboring transmitters has formed a problem to which particular attention has been paid from the outset. If fact, in a radar the duplexers which are limiters inserted in the receiving chain between two 3 dB junctions function, in the case where the equipment operates with a single common transmitting-receiving antenna, to decouple considerably the transmitter from the receiver by at least 40 decibels.
The role and the operation of a duplexer in a radar connected to a single antenna will be recalled briefly. It is absolutely necessary to avoid high-level energy imparted by the transmitter to the antenna from reaching the receiver which it would destroy. The same goes for the energy transmitted from a radar adjacent the one concerned. Conversely, it is extremely desirable that any energy collected by the antenna from a target illuminated by the transmitted radiation be passed on without loss to the receiver. The duplexer also plays the role of a switch isolating the receiver during transmission or during a powerful adjacent transmission and opening the channel during reception.
The protection of radar receivers against high powers from their transmitter or that of neighboring transmitters was first of all provided by gas tubes, called TR tubes which, inserted into the receiving chain had the structure of a pass-band filter and the property of being passing when the ultra-high frequency energy applied was low and reflecting when this energy exceeded a few watts peak. These tubes (which moreover gave satisfaction) presented the drawbacks due to limited life expectancy and a slight delay on ignition, thus letting through a part of the power towards the receiver which finished by being damaged.
The appearance of P.I.N. diodes enabled in some cases these gas tubes to be replaced by switches which are more reliable as regards their life expectancy. However, it is necessary to associate with these PIN diodes a control circuit for enabling them a little before transmission by means of an auxiliary biasing source and to apply thereto an inverse voltage so as to reduce losses at the moment of reception. Since these control circuits are synchronous with the radar transmission, the PIN diode switches do not however protect the radar receiver against neighboring transmitters.
Finally, the use of specially-designed diodes associated in pairs has made possible the construction of a solid-state protection device, entirely passive and able to withstand powers of about ten kilowatts peak. However, despite the fact of mounting the diodes in cases having a ceramic part made from a good heat-conducting material, such as Beryllium oxide, the heating caused by their losses, although very small, leads to their destruction when the transmission power becomes too great. Indeed, the diodes are in a zone where coupling is maximum. Another drawback concerning the use of these diodes for devices usable as high-power wave-guides and at low frequencies is that the dimensions of the ultra-high frequency structures are very large compared with the dimensions of the diodes, thus making it difficult to optimize the performances thereof.